Republican strategist Chip Saltsman announced he has resigned from the campaign of Joe Carr — just hours after the candidate switched from a House race to challenge Sen. Lamar Alexander, R-Tenn., in the primary.

"I signed up to help you run for Congress, not the Senate," Saltsman wrote in his letter of resignation, which the operative released to the press Tuesday morning.

Carr, a state representative, initially launched a campaign to challenge embattled Rep. Scott DesJarlais in the 4th District GOP primary. However, that race was getting crowded, with state Sen. Jim Tracy also vying for the nomination and so far leading in the fundraising battle.

Saltsman signed up with Carr's House campaign three months ago, when Carr called his experience "invaluable." Saltsman is a former state party chairman and campaign manager for former Arkansas Gov. Mike Huckabee's presidential bid in 2008.

In his letter, Saltsman noted Alexander's dogged support for Republicans up and down the ballot, including helping keep Vice President Al Gore from winning his home state in 2000.

"It is because of Lamar Alexander that people like you have the honor of serving in the majority of the state legislature," Saltsman said.